Control servomechanisms are commonly utilized in conjunction with a wide variety of fluidly-controlled devices, such as pneumatic cylinders, hydraulic cylinders, fuel metering valves, and the like. One known control servomechanism, commonly referred to as a “force balance servo” or a “nut cracker servo” and referred to herein as a “torque balance servo,” includes a roller assembly that is mechanically linked to a translatable piston mounted in the fluidly-controlled device. The roller assembly contacts and exerts a force on a pivoting table. Although normally residing in a null position, the pivoting table may pivot about a rotational axis when the forces biasing the pivoting table to rotate in a first direction (e.g., clockwise) exceed those biasing the table to rotate in the opposite direction (e.g., counter-clockwise). The pivoting table is positioned near an outlet nozzle, which is fluidly coupled to a variable-pressure chamber provided in the fluidly controlled device. As the pivoting table pivots about its rotational axis, a cap attached to the pivoting table selectively impedes fluid flow through the outlet nozzle. When the cap resides adjacent the outlet nozzle and substantially impedes fuel flow therethrough, the pressure within the variable-pressure chamber increases. Conversely, when the cap resides further away from the outlet nozzle and does not substantially impede fuel flow, the pressure within the variable-pressure chamber decreases. The translational position of the piston is generally determined by the pressure within the variable-pressure chamber and, thus, the angular position of the pivoting table. After the translational position of the piston has been adjusted in this manner, the torques exerted about the rotational axis of the pivoting table again reach a state of equilibrium and the pivoting table returns to the null position.
Conventionally, torque balance servos of the type described above utilize a mechanical force bias mechanism to temporarily adjust the angular position of the pivoting table and, therefore, the translational position of the piston contained within the fluidly-controlled device (e.g., the piston of a fuel metering valve). Such systems are traditionally mechanical in nature with the piston following the mechanical input in a known and consistent relationship returning to the null position after motion ceases.
In traditional systems requiring electrical input, the electrical input relationship to position or velocity of the output device (e.g., the piston of a fuel metering valve) depends on a varying gap in a torque motor, a proportional solenoid, or other such device that controls flow to the output device. These devices generally fail to provide a consistent relationship between the current applied to the position or velocity of the translatable piston within the fluidly-controlled device because consistent relationship between current and gap width cannot be maintained from unit to unit or over wide variations in temperature. Although an electrical feedback transducer (e.g., linear variable differential transducer) may be employed to continually monitor piston position and thus compensate for servo inconsistencies, the provision of such a feedback transducer adds undesirable cost, weight, and complexity to the system.
Considering the above, it would be desirable to provide a torque balance servo that achieves a highly accurate current-to-position output without the use of an electrical feedback transducer. Preferably, such a torque balance servo would maintain the accuracy of the current-to-position output over a relatively wide range of operating temperatures. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended claims, taken in conjunction with the accompanying drawings and this Background.